Topic 9 Flashcards
How do you convert from Celsius to Kelvin
Add 273
Why is the absolute scale used
It doesn’t arbitrarily depend on the properties of a given substance.
0K (absolute zero) means that the particles have minimum internal energy
Describe the arrangement and energy of particles in a solid, a liquid and a gas
Solid- regular arrangement, vibrate around fixed positions
Liquid - close together, constantly moving past each other
Gas - spaced very far apart, free to move in all directions
How does Brownian motion give evidence for the particle model of matter
Smoke particles suspended in air can be seen to move randomly in all directions. This must be as a result of random collisions with particles making up the air
What is internal energy
The sum of the potential and kinetic energies of a system
True or false: At a given temperature, all particles in a material have the same kinetic energy
False. The kinetic energies will be randomly distributed around a central ‘most likely’ amount
How can you increase the thermal energy of a system
We can increase it by heating it up or doing work on the object
Explain the energy changes that occur during a change of state
During change of state the potential energy of the particles change but the kinetic energies don’t change
What equation can be used to determine the energy required to change the temperature of a substance
E = mcΔθ
Where E = energy, m = mass, c = specific heat capacity, Δθ = temperature change
What is the specific heat capacity of substance
The energy required to raise the temperature of 1kg of a substance by 1K
Give the equation to workout the energy for a change of state
E = ΔmL
Where E = energy, Δm = mass change, L = specific latent heat (‘of fusion’ if melting/freezing, ‘of vaporisation’ if condensing/evaporating)
What is the specific latent heat of a substance
The energy required to change the state per unit mass of a substance, while keeping the temperature constant
In an experiment to find ‘c’ for water, lots of energy input escapes to the surrondings. Will this lead to an over or underestimate of specific heat capacity
- An overestimate
- Specific heat capacity is calculated as: c = E / (mΔθ)
- The energy input will be used, but the temperature change of the water will be lower than it should be due to the escaped energy - therefore c will be too high
What is Avogadro’s constant (in words)
The number of atoms there are in one mole of a substance
How is the motion of gas molecules described
Gas molecules move with Brownian motion, which is the random motion of molecules caused by collisions with larger particles
How does a gas exert a force on its container
- The molecules collide with the walls of their container
- Collisions cause a change in momentum
- A change in momentum produces a force equal to the rate of change of momentum
What does Boyle’s law state
When a gas is at a constant temperature, pressure and volume are inversely proportional to each other
Explain Boyle’s Law
- When the volume of a gas increases, the space between molecules increases and so the time between collisions is larger.
- This causes the rate of collisions and so the rate of change of momentum decreases.
- This means the force exerted is lower, causing a decrease in pressure
What does Charles’ law state
When a gas is at a constant pressure, the volume is directly proportional to the absolute temperature
Explain Charles’ Law
- As temperature increases, the average Kinetic energy of the molecules increases
- Pressure is constant so the force and so also the rate of change of momentum, must remain constant
- To achieve this, the volume increases so the faster speed of the molecules is compensated by there being larger gaps between them
What does the Pressure Law state
When a gas has a fixed volume, pressure is directly proportional to the absolute temperature
Explain the Pressure Law
- As temperature increases, the average kinetic energy, and so the speed of the molecules also increases
- This increases the rate of collisions, and so produces a larger rate of change of momentum
- This leads to a greater force exerted and so an increase in pressure
In kinetic theory, what is assumed about the gasses involved
- The gas contains a large number of molecules
- The molecules are identical to each other
- All collisions between molecules and the walls of their container are perfectly elastic
- The time taken for collisions is negligible compared to the time between collisions
- There are no intermolecular forces between molecules
- Molecules are in constant random motion
- The gas particles obey Newton’s Laws of motion
What follows all the assumptions made in kinetic theory
Ideal gases
State the relationship between volume and pressure for a given quantity of gas at a fixed temperature
Pressure is inversely proportional to volume
P = k/V
State the relationship between volume and temperature for a given quantity of gas at a fixed pressure
Volume is directly proportional to temperature
V = kT
State the relationship between pressure and temperature for a given quantity of gas with a fixed volume
Pressure is directly proportional to temperature
P = kT
Combine the relationships between pressure, volume and temperature into a single expression, with a constant
(pV)/T = constant
State the ideal gas equation
pV = NkT pV = nRT
What does ‘k’ represent in the ideal gas equation
THe Boltzmann constant (= 1.38x10^-23 J/K)
What does ‘N’ represent in the ideal gas equation
The number of molecules
How do you convert between the number of moles and number of molecules in a sample
Multiply the number of moles by the Avagadro constant
State three ideal gas assumptions about the motion of the particles in an ideal gas
- The particles obey Newton’s Laws of motion
- They travel in straight lines
- They are in random motion
What assumption is made about the forces between molecules in an ideal gas
There are no intermolecular forces acting between collisions
What can be said about the internal energy of an ideal gas
There are no intermolecular forces and so there is no potential energy. This means the internal energy is entirely kinetic energy
Describe the collisions between the molecules in an ideal gas
The collisions between molecules and between molecules and the container are elastic
What assumptions are made about the size of the molecules in an ideal gas
- The volume of the molecules is negligible in comparison to the volume of the container
- All the molecules are identical
State the equation linking the pressure and volume of a gas to the speed of its molecules
pV = 1/3 Nm
What is Wien’s Law
Peak wavelength x Temperature = 2.898x10^3
λxT = 2.898x10^3
Peak wavelength is the wavelength at which the emitted radiation is most intense i.e. there is a peak on the black body radiation curve
What is the peak wavelength
The peak wavelength is the wavelength at which the emitted radiation is most intense i.e. there is a peak on the black body radiation curve
What is the Stefan-Boltzmann law equation
L = σAT^4
L is power output (W), σ is the Stefan-Boltzman constant, A is the surface area of the black body (m^2) and T is te temperature (K)
What equation can be used to calculate the kinetic energy of a molecule from its temperature
EK = 3/2 kT
k is the boltzmann constant and T is the temperature (K)
What is meant by the root mean square speed
The square root of the mean of the squares of the speeds of the molecules
What is an ideal gas
A gas where:
- The gas molecules don’t interact with each other
- The molecules are thought to be perfect spheres
